ABSTRACT

Current explanations for low-angle, younger over older faults in eastern Nevada are (1) a master detachment extending through the crust, (2) mid-crustal ductile extension, and (3) local gravity sliding. None of these explanations are fully supported by field evidence.

Our field evidence indicates that thermally induced ductile extension beneath the White Pine and Schell Creek ranges produced low-angle, normal faults in competent units above and detachment faults along ductile shales. Thin brittle units between shale detachments, fragmented by low-angle normal faulting, were pulled apart into isolated lenticular detachment structures. Duck Creek Valley, a Neogene graben similar to Railroad Valley, may have formed in a zone of weakness caused by Oligocene attenuation, suggesting that potential oil-producing basins in Nevada may contain subsurface detachment structures.

Oligocene attenuation may have contributed to fracturing of reservoirs (brittle units), local thinning of source rocks (ductile units), and formation of structural traps on detachment topography. Lateral variation in fracture density, by creating barriers to updip migration, also may contribute to oil accumulation. Other favorable structures could be large, early Miocene gravity slide blocks sealed by Neogene basin fill. In basins similar to Railroad Valley, these structures should be prime exploration targets because oil generation occurred after the structures formed.

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